In an internal combustion engine, it is known to use a fuel injector to deliver a charge of fuel into the cylinders of the engine. One type of fuel injector that is particularly suited for precise metering of fuel is a so-called piezoelectric injector. Such an injector allows precise control of the timing and total delivery volume of a fuel injection event. This permits improved control over the combustion process which is beneficial in terms of exhaust emissions.
A known piezoelectric injector 2 and its associated electronic control system 3 is shown schematically in FIG. 1. The piezoelectric injector 2 includes a piezoelectric actuator 4 that is coupled, by way of a hydraulic amplifier 5, to an injector valve needle 6 in order to control the position of the valve needle 6 relative to a valve needle seat 8. As known in the art, the piezoelectric actuator includes a stack 7 of piezoelectric elements that expands and contacts in dependence on the voltage across the stack 7. The axial position, or ‘lift’ of the valve needle 6 is controlled by applying a variable voltage ‘V’ to the piezoelectric actuator 4 to control the voltage across the stack 7, thus controlling the transfer of electrical charge applied to or removed from the stack 7. Although not shown in FIG. 1, it should be appreciated that, in practice, the variable voltage would be applied to the actuator 4 by connecting a power supply plug to the terminals of the injector 2.
By application of an appropriate voltage across the actuator 4, the valve needle 6 is caused either to disengage the valve needle seat 8, in which case fuel is delivered into an associated combustion chamber (not shown) through a set of nozzle outlets 10, or is caused to engage the valve needle seat 8, in which case fuel delivery through the outlets 10 is prevented.
The piezoelectric injector 2 is controlled by an injector control unit 20 (hereinafter ‘ICU’) that forms an integral part of an engine control unit 22 (hereinafter ‘ECU’). The ECU 22 monitors a plurality of engine parameters 24 and calculates an engine power requirement signal (not shown) which is input to the ICU 20. In turn, the ICU 20 calculates a required injection event sequence to provide the required power for the engine and operates an injector drive circuit 26 accordingly.
In order to initiate an injection, the injector drive circuit 26 causes the differential voltage between the high and low voltage terminals of the injector, V1 and V2, to transition from a high voltage (typically about 200 V) at which no fuel delivery occurs, to a relatively low voltage (typically about 50 V), which initiates fuel delivery. An injector responsive to this drive waveform is referred to as a ‘de-energise to inject’ injector and is operable to deliver one or more injections of fuel within a single injection event. For example, the injection event may include one or more so-called ‘pre-’ or ‘pilot’ injections, a main injection, and one or more ‘post’ injections. In general, several such injections within a single injection event are preferred to increase combustion efficiency of the engine.
The amount of charge applied to and removed from the piezoelectric actuator can be controlled by two methods. In a ‘charge control’ method, current is driven into or out of the piezoelectric actuator for a period of time so as to add or remove, respectively, a demanded amount of change to or from the stack, respectively. Alternatively, in a ‘voltage control’ method, a current is driven into or out of the piezoelectric actuator until the voltage across the piezoelectric actuator reaches a demanded level. In either case, the voltage across the piezoelectric actuator changes as the level of charge on the piezoelectric actuator varies, and vice versa.
For further background to the invention, an injector of this type is described in the applicant's European Patent No. EP0955901B. Such fuel injectors may be employed in compression-ignition (diesel) engines or spark ignition (petrol) engines.
FIG. 2 is a graph that shows the actuator voltage (or ‘drive pulse’) during a main injection when driven by the injector drive circuit 26. The actuator voltage includes a discharge phase (between time period T1 and T2), a dwell period (between T2 and T3), and a charge phase (between T3 and T4). It should be appreciated, however, that such an injection could also include one or more ‘pre-’ or ‘pilot’ injection events, and/or one or more ‘post’ injection events.
Piezoelectric injectors provide accurate control over the volume of fuel that is delivered during an injection event and enable precise repeatability between one injection event and another. However, at present piezoelectric injectors are operated to open and close without any information regarding the axial position of the valve needle. Therefore, in order for the ECU to define the instant of the start and the end of an injection event, the ECU is required to perform calculations to compute the necessary duration of the injector drive pulse to deliver a given volume of fuel. The calculated injector drive pulse duration is based on the known dynamic behaviour of the valve needle and the prevailing engine operating conditions such as injection fuel pressure, fuel temperature, engine speed and the like. However, the accuracy with which the ECU is able to calculate the onset of valve needle movement following an injector discharge phase is highly susceptible to the effects of actuator aging and this presents a problem to the long term viability of a piezoelectric injector.
There is therefore a need to provide a means to feedback information to the ECU of a vehicle regarding the operation status of the fuel injectors so that the effects of piezoelectric aging of the actuator of the injector can be mitigated.